The invention relates to a rotary-wing aircraft rotor with constant velocity drive, in particular for a convertible aircraft supporting two generally fixed wings and at least one tilting rotor.
Although the constant velocity drive rotor of the invention can be used as a helicopter rotor, in particular as a tail or anti-torque rotor, a particularly advantageous application of such a constant velocity drive rotor consists in fitting it to convertible aircraft with tilting rotors, particularly of the type known and described in FR 2 791 319, FR 2 791 634 and FR 2 798 359, to which reference may advantageously be made for further details.
Briefly, such a convertible aircraft with tilting rotors generally comprises, as shown schematically in FIG. 1, a fuselage 1, of the aircraft fuselage type, supporting two fixed wings 2, for example high wings, raised with respect to the fuselage 1, each wing 2 itself supporting a power pod 3, housing a power unit driving in rotation a rotor 4, represented schematically by the plane of rotation of the rotor blades, via a transmission (not shown in FIG. 1), a rear reduction gear unit of which is directly driven by the power unit and housed like the latter in the stationary rear part 5 of the power pod 3. The front part 6 of the power pod 3, housing a rotor mast and a rotor hub, as well as a front reduction gear unit driving the rotor mast in rotation, is mounted so as to pivot with the rotor 4, so that it can pivot from an aeroplane configuration, in which the rotor 4 operates as a propeller at the front of an engine pod 5-6 facing into the relative wind, to a helicopter configuration, in which the rotor 4xe2x80x2 operates as a helicopter main lifting rotor at the upper end of the front pivoting part of the pod in the upright position 6xe2x80x2 above the corresponding wing 2, this latter configuration being used for take-off and landing of the convertible aircraft which, after transition from the helicopter configuration to the aeroplane configuration, is able to move in forward flight like an aircraft.
As a variant, the pods 3 may pivot totally with the rotor 4 with respect to the fixed wings 2.
On rotary-wing aircraft rotors, it is known that, since the introduction of the flapping hinge on autogyro and helicopter rotors, tilting the rotor where coning is present, whether this tilting is desired and generated by controlling the cyclic pitch or the unwanted result of the asymmetry between an advancing blade and a retreating blade, causes stresses in the drive plane of the blades which tend to cause the blades to accelerate and decelerate in the course of a revolution of the rotor. These variations in speed are caused by Coriolis forces, and may be illustrated simply by the fact that the trajectory of the blade tips, viewed in a plane perpendicular to the drive axis, is an eccentric ellipse, the angular rate of travel of which is constant and, consequently, the peripheral speed of which varies over a revolution. This acceleration and deceleration of the blades over a revolution of rotation has a disastrous effect on the lives of the rotor components, due to the fact that these variations in speed generate stresses which are all the more substantial as the rigidity of the rotor components is high.
Conversely, it is known that great flexibility about the drag axis of the blades has a highly beneficial effect on the dynamic stresses to which the blades and the components of the rotor hub are subjected, which is why the introduction of the flapping hinge has been accompanied by the introduction of the drag hinge.
These improvements to the original rotary-wing aircraft rotor concepts have led to a rotor fully articulated in pitch, flapping and drag, the main disadvantage of which was to be subject to instability of the ground resonance or air resonance type, which made it necessary to develop and use drag dampers, also known as frequency adapters, or again elastic return drag struts with built-in damping. On helicopter rotors, these drag dampers are arranged in the plane of rotation of the rotor, between the blades and the hub of the rotor in a conventional configuration, or between adjacent blades of the rotor in the inter-blade configuration. In both cases, the presence of the drag dampers increases the aerodynamic drag of the rotor, in particular at the hub and where the hub is connected to the blades, which reduces the overall performance of the helicopter.
On a convertible aircraft of the tilting rotor type presented above, in which the speed of travel in the aeroplane mode is far higher than that of a helicopter, and on which drag dampers, mounted as on a helicopter rotor, would be head on to the wind, this reduction in performance would be far more appreciable, which is why designers of convertible aircraft of this type have endeavoured, for the design of the rotors, to retain hubs which are extremely rigid in drag (known as stiff-in-plane rotors), with no drag dampers, the natural drag frequency of which is greater than the nominal frequency of rotation of the rotor, which eliminates any risk of instability in drag, even in the absence of drag dampers.
However, it is known that rotors which are rigid in drag have the major disadvantage of generating very high stresses when the rotors are tilted. On convertible aircraft, the importance attached to producing rotors of high aerodynamic efficiency, and therefore with no drag dampers, has led to the development of hubs which are not sensitive to Coriolis forces. A particular feature of these hubs, which include hubs with a universal joint drive, is that tilting of the rotor is accompanied by tilting of the drive axis of the latter. Because of this, the rotor drive axis is always perpendicular to the rotor plane, and the trajectory described by the blades always remains a circle in a plane perpendicular to the drive axis of the rotor. This type of drive has been used, for example, on prototype convertible aircraft, particularly the XV15 aircraft.
However, a known particular feature of universal joints is that they are not of the constant velocity type, which manifests itself by the fact that the output speed of these joints is not always equal to the input speed. This speed distortion occurs when the drive and output axes are not co-linear, i.e. in the application considered to driving a rotor in rotation, when cyclic flapping is present. In the simplest configuration of a universal joint, the latter comprises a spider, the joints of which, by one arm of the spider to a driving shaft and by the other arm of the spider to a driven shaft, allow the driven or output shaft to swivel relative to the driving or input shaft. It is known that these speed variations caused by such a universal joint, and transmitted to the driven shaft, correspond to acceleration and deceleration which, over one revolution of rotation of the universal joint, appear twice. The speed of the driven shaft is therefore not constant, but varies at a frequency equal to twice the frequency of rotation of the shafts.
To eliminate these speed variations, which are responsible for very substantial inertial forces, in the case of a rotary-wing aircraft rotor, which affect the hub as a whole and are prejudicial to the durability of the mechanical assemblies constituting the hub or associated with the latter, several constant velocity drive systems have been proposed, particularly so-called Clemens drive links, composed of assemblies of two branches hinged respectively to the driving and driven shafts and connected by a swivel, and also tripod joints, for which transmission of movement is provided by means of balls moving in axial grooves machined in the driving and driven shafts.
These arrangements are used to ensure that the drive point is always situated in a plane bisecting the axes of the driving and driven shafts. As the distances from this point to the axes of the two shafts are then identical, the speeds of rotation of the two shafts are strictly equal whatever the angular position of the two shafts, which guarantees that the transmission provides a constant velocity drive.
These two known constant velocity drive systems are not suitable for application to convertible aircraft rotors for the following reasons:
installing Clemens drive links on a convertible aircraft rotor hub very substantially increases the drag of the hub, which reduces its performance and increases operating costs;
tripod joints are not suitable in particular because of the high torque levels encountered on convertible aircraft rotor hubs, which require large diameter and therefore heavy balls to keep the contact surface Hertz pressures at acceptable levels.
In other arrangements, the swivelling and drive functions are kept separate. This is the case in the constant velocity drive system of the V22 tilting rotor convertible aircraft, in which the swivelling function is provided by two halves of a spherical laminated flapping thrust bearing enclosing the hub and connected to the rotor mast. This function absorbs the lift and the coplanar loads due to the aerodynamic and inertial excitation of the rotor. The mast drives the rotor (transmits the torque) by three drive links each connected at one end to the hub and at the other end to the mast.
A variant of this system is proposed in patent U.S. Pat. No. 5,145,321, in which the drive function is provided by substantially parallelepiped-shaped swivel bearings.
A particular feature of these separate means providing the swivelling and drive functions of the hub relative to the rotor mast is that they are kinematically not compatible in the absence of flexibility of the elements connecting the hub to the mast, and constant velocity drive is obtained only by careful tailoring of the rigidity of these connecting elements. Where cyclic flapping of the rotor is present, each drive link mentioned above is subject to dynamic stress at a frequency equal to twice the frequency of rotation of the rotor, the phase depending on the position of this link relative to the hub. For regularly spaced links, in a circumferential direction about the axis of rotation, the phase difference between the dynamic loads on the links is such that the contributions to the dynamic torque cancel each other out, which is a necessary and sufficient condition for constant velocity drive of the hub by the rotor mast.
Another major disadvantage of this type of hub in addition to the disadvantages regarding the need for flexibility of the connecting elements and for accurate tailoring of drive link rigidity is that the enclosing arrangement of the halves of the flapping thrust bearing make it difficult to inspect the links in particular and the torque transmission system in general, as well as hampering access to these for maintenance purposes.
In patent U.S. Pat. No. 5,145,321 mentioned above, the vertical shear of a substantially parallelepiped-shaped swivel bearing allows the rotor to tilt about an axis perpendicular to the axis joining the centre of the bearing to the rotor drive axis. Movement of the rotor about a second pivot axis is made possible by the ball joint fitted inside the parallelepiped-shaped bearing. In the same way as for a system where the hub is driven by the mast via links, as presented above, a minimum number of three bearings with closely similar levels of rigidity is required to obtain a constant velocity drive. The flexibility required for correct operation is also directed according to the direction of drive in rotation.
The problem addressed by the invention is to propose a constant velocity drive rotary-wing aircraft rotor, in particular for a convertible aircraft with at least one tilting rotor, all of the constant velocity drive means and tilting means of which have the following degrees of freedom:
two degrees of freedom in rotation about two coplanar axes, secant, for tilting of the hub and therefore of the rotor,
no degrees of freedom in translation, which is equivalent, in terms of loads, to:
the loads, i.e. lift and the coplanar loads, being applied along the two axes considered above and along the axis about which the drive in rotation takes place, and:
the moments being applied about the axis of rotation of the rotor mast only, which corresponds to the drive torque of the hub, the swivelling capability of these means being therefore only partial, since they offer no freedom of rotation of these means about the mast, the rotor of the invention providing a solution to the disadvantages of state-of-the-art rotors of this type, and such as presented above, and in particular not requiring the presence of laminated connecting elements produced with accurately tailored rigidity to obtain a constant velocity drive, while making the swivelling and driving means kinematically compatible, whether these means are separate or merged, and even in the absence of a specific flexibility provided by laminated connecting elements.
To this end, the invention proposes a rotary-wing aircraft rotor with constant velocity drive for a convertible aircraft with at least one tilting rotor, comprising:
a rotor mast, capable of being driven in rotation about its longitudinal axis,
a hub, connected to said mast by a constant velocity drive mechanism and by a tilting arrangement, allowing the hub as a whole to tilt about any flapping axis converging with the axis of the mast and perpendicular to said axis of the mast, so that said hub is capable of being driven in constant velocity rotation by said mast, about a geometrical axis of rotation of the hub which may be inclined in any direction about the axis of the mast, and
at least two blades, each linked to said hub by a coupling retaining and hingeing its blade in pitch,
wherein said constant velocity drive mechanism comprises a differential mechanism for splitting static torque and allowing relative movement, in a plane perpendicular to said axis of the mast, between at least two devices for driving the hub, said differential mechanism comprising a set of three discs positioned substantially one above another and substantially coaxial about said axis of the mast, and of which a first disc, arranged between second and third discs along said axis of the mast, is a driving disc, integral in rotation with said mast, and connected to each of the second and third discs, which are driven, by at least one connecting pin, having a longitudinal geometrical axis substantially parallel to said axis of the mast, and hinged to each disc in the set by one respectively of three ball joint connections substantially centred on the longitudinal geometrical axis of said connecting pin, each of the second and third discs being connected to the hub by one at least of said at least two driving devices, which are each also hinged to the hub, so as to drive it in rotation about said geometrical axis of rotation of the hub.
According to a first embodiment, the second disc (hereinafter referred to as a first of the driven discs) drives in rotation, about said axis of the mast, and preferably via two drive pins coaxial about a first diametral axis of said mast, a first driving device arranged as a first gimbal, mounted so as to pivot about said first diametral axis, which is substantially perpendicular to said axis of the mast, by two first bearings diametrically opposite relative to said axis of the mast; and the third disc (hereinafter referred to as a second of the driven discs) drives in rotation, about said axis of the mast, and preferably also via two drive pins coaxial about a second diametral axis of the mast, a second driving device, arranged as a second gimbal, mounted so as to pivot about the second diametral axis, which is substantially perpendicular to said axis of the mast and to said first diametral axis, and converging with this latter on said axis of the mast, by two second bearings diametrically opposite relative to said axis of the mast, said first gimbal being in addition hinged to the hub by two first ball joint connections, diametrically opposite relative to said axis of the mast, and each centred substantially in a plane defined by said axis of the mast and said second diametral axis, and said second gimbal being in addition hinged to said hub by two second ball joint connections, diametrically opposite relative to said axis of the mast, and each centred substantially in a plane defined by said axis of the mast and said first diametral axis, so that the gimbals, their pivot bearings on the driven discs and their ball joint connections hingeing them to the hub constitute the arrangement for tilting of the hub as a whole whilst belonging to the constant velocity drive mechanism of the hub.
In this embodiment, the rotor according to the invention comprises means of driving and hingeing the hub by and relative to the mast which are based on a universal joint of which the two successive hinges would be combined at the same location between the driving body, the rotor mast, and the driven body, the hub, in such a way that this device has the advantage of simultaneously performing the two functions of swivelling and torque transmission by means of a small number of parts, which makes it relevant in terms of weight, cost and maintenance.
The main advantage of such a rotor according to the invention, with its three-disc differential mechanism, compared with a similar rotor not equipped with such a mechanism, is that it enables the drive means to provide a constant velocity drive, without the need to define connecting elements having specific flexibility and substantially the same torsional rigidity in the two torque transmission trains connecting the mast to the hub, and each passing through one respectively of the gimbals.
In fact, in order that the swivelling means and the tilting means should be compatible kinematically, it is necessary for the two gimbals to be able to perform small relative angular deflections about the geometrical axis of rotation of the hub. This is the result of the fact that where the hub is tilted relative to the mast and about an axis not converging with the pivot axes of the gimbals, pivoting of the gimbals in the absence of flexibility between the two torque transmission trains causes rotation of the gimbals in opposite directions about the drive axis of the rotor. Pivoting of one of the gimbals tends to cause the hub to advance, in the direction of rotation of the rotor, whereas pivoting of the other gimbal tends to cause the hub to retreat (rotating in the opposite direction to the direction of rotation of the rotor). To escape from this hyperstatic state, an additional degree of freedom is introduced along the drive axis, and this is obtained precisely by means of the three-disc differential mechanism, allowing splitting of the static torque transmitted from the mast to the two gimbals and relative movement of the two gimbals. In fact, any tilting of the rotor and its hub on the mast induces a relative cyclic rotation of the two gimbals at a frequency of 2 xcexa9, which is compensated for kinematically by the connecting pins hinged to the three discs, and allowing rotation of the driven discs in opposite directions relative to the driving disc, and about the axis of the mast.
The presence of this differential mechanism means that it is no longer necessary to use laminated connections with tailored rigidity on the swivel bearings and ball joint connections of the gimbals. However, it is advantageous for the pivot bearings of the gimbals on the driven discs and/or the ball joint connections hingeing the gimbals to the hub to comprise cylindrical, spherical or truncated cone-shaped elements, or a combination of such elements with the aim of reducing the friction induced by the kinematics of the two gimbals and thus increasing the lives of the components.
According to a second embodiment, each of the driven discs drives in rotation about said axis of the mast, via two drive pins, two driving devices arranged as links, diametrically opposite and aligned substantially tangentially relative to said axis of the mast, the drive links being regularly arranged around said axis of the mast, so that each of the two links driven by one of the driven discs is between the two links driven by the other driven disc, each drive link being hinged at one of its ends to one respectively of the two drive pins of one respectively of the driven discs and, at its other end, to an end fitting attaching it to the hub.
The advantage of such a rotor with a differential mechanism connected to the links providing a drive function kept separate from the swivelling function, compared with rotors of the same type with no differential mechanism, is that the constant velocity characteristics of the drive are not obtained by the generation of large out-of-phase loads which are cancelled out when added together, but because of the kinematic compatibility introduced by the differential mechanism. Accurate tailoring of the rigidity of the links is not necessary, which greatly simplifies the design of the latter.
However, each drive link is still advantageously equipped at each of its two ends with a ball joint connection, preferably comprising a laminated ball joint, for hingeing one end of said link to a drive pin of a driven disc, and hingeing the other end of said link to an end fitting for attaching to the hub, in order to relieve the links of any superfluous stresses in their angular deflection relative to the driven discs, on the one hand, and to the hub on the other, when the latter is tilted.
In this second embodiment, the swivelling function, kept separate from the drive function, may be provided in a manner known in itself, by at least one half of a central laminated spherical flapping thrust bearing, and preferably by two halves of a thrust bearing of this type which enclose the central part of the hub and the drive means, each half of the thrust bearing having at least one member connected to the hub and at least one member integral in rotation with the mast.
The spherical flapping thrust bearing thus constituted transmits to the rotor mast the lift and coplanar loads applied to the rotor.
However, even in the first embodiment, with a double gimbal combined with a differential mechanism, in order to improve the rigidity of the rotor in cyclic flapping, the hub may also and advantageously be connected to the mast by at least one elastic return member returning the hub to a rest position substantially perpendicular to the axis of the mast, and in a manner known in itself, these elastic return member may advantageously comprise at least one half of a central laminated spherical thrust bearing, of which at least one part is connected to the hub and at least one other part is integral in rotation with the mast. This half of a spherical thrust bearing may be fitted under the central part of the hub, the constant velocity drive mechanism and the tilting arrangement and, if in addition the elastic return member also comprises an upper half of a central laminated spherical thrust bearing, which covers and encloses the central part of the hub, a central laminated spherical thrust bearing is then obtained which assists in transmitting to the rotor mast the lift and coplanar loads applied to the rotor.
In the different embodiments, for proper distribution of the loads involved in connecting the driving disc to the two driven discs in order to ensure proper kinematic compensation, it is advantageous for the differential mechanism to comprise at least two pins connecting the three discs, said connecting pins being arranged regularly in a circumferential direction about said axis of the mast and, more particularly at least two assemblies of at least two adjacent connecting pins per assembly, said assemblies of connecting pins being regularly distributed in a circumferential direction about said axis of the mast. To facilitate limited amplitude rotation in opposite directions of the driven discs relative to the driving disc and to the mast, each connecting pin is advantageously hinged in the driving disc by a central ball joint connection of larger diameter than the diameter common to the two end ball joint connections of said connecting pin, and by each of which said connecting pin is hinged in one respectively of the two driven discs. According to a simple structure, each connecting pin is advantageously a triple ball joint pin.
To improve their strength, these connecting pins, subject to torque loads, may be laminated and advantageously exhibit a certain flexibility along their longitudinal geometrical axis, in order to allow even distribution of the loads passing through each connecting pin, but this flexibility is not critical for the transmission to be of the constant velocity type. It is also necessary in a radial direction, relative to the axis of the mast, for kinematic reasons. To this end, each of the three ball joint connections of each connecting pin is preferably laminated and also comprises a cylindrical laminated bearing, substantially coaxial with said connecting pin.
For transmitting the lift load and the coplanar loads from the hub to the mast, each driven disc is advantageously mounted axially between two radial annular bearings, surrounding said mast and substantially coaxial about said axis of the mast, and allowing rotation, about said axis of the mast, of each of said driven discs relative to said mast and to the driving disc and, moreover, at least one axial bush is preferably mounted between a peripheral and axially offset portion of each driven disc and the driving disc, in order to allow relative rotation, about said axis of the mast, of the driven discs relative to the driving disc and to said mast.
These radial annular bearings and/or axial bushes may be plain, but advantageously comprise cylindrical and/or truncated cone-shaped and/or spherical laminated elements.
In a simple manner, for transmission of the torque, the driving disc is made integral in rotation with said mast by internal axial splines engaged with external axial splines on an end portion of said mast, axially at the opposite end to the base of said mast, by which the latter is driven in rotation.
In addition, a first of the two driven discs may be arranged axially between the driving disc and an outer radial shoulder integral in rotation with said mast, while the second driven disc is arranged axially between the driving disc and a device for axial preloading of the assembly of the three discs, and mounted on the free end of said mast.
In the different modes of embodiment, it is advantageously simple for each driven disc to support two drive pins diametrically opposite relative to said axis of the mast, and by which the corresponding driven disc is connected to one at least of said driving devices, the drive pins of the two driven discs extending substantially in the same plane perpendicular to the axis of said mast.
In order that the hub may advantageously be rigid in its plane, and that all of the constant velocity drive means and, where appropriate, the tilting means are suitably protected, the devices for driving the hub in rotation from the driven discs are advantageously connected to a hub casing, which surrounds said driving devices and said assembly of three discs, and is attached to a hub plate connected to the blades and having a central opening through which said mast runs. The hub plate may then be a plate of known type, of composite material, and in the form of a star with outward-extending arms equal in number to the number of the blades and on each of which are mounted the coupling for retaining and hingeing a blade in pitch, this arrangement providing good rigidity in drag and a certain flexibility along the flapping axis.